Why you should replace automatic transmission fluid on time

Over a period, the life of automatic transmission fluid reduces if the operating temperature exceeds the specified range for longer durations.

BHPian Indian2003 recently shared this with other enthusiasts.

Here is an interesting note about automatic transmissions and temperatures. There is a guy who repairs gearboxes down the road from my place. I might pop in tomorrow and visit him.

A few years ago, he used to make torque converters which he delivered to shops in Norway and Sweden.

The original article is in Spanish and I had asked our friend Google for some help with a little input from me.

Automatic Transmission

Most user manuals state that the automatic transmission is filled for life. Below we will give the reasons why the ATF should be replaced at, on average, every 40,000 km.

In an automatic transmission, a lot of heat is generated due to the friction within the torque converter, when the clutch plates engage, and that is created in the gears and rotating guides. When this heat is produced, the ATF quickly warms up once the vehicle is in motion.

Under normal driving conditions, the fluid temperature reaches 80°C (175°F), which is the average range in which most liquids are designed to operate. If it is kept at that temperature, the fluid can last up to 160,000 km, but if the temperature rises, (even under normal driving conditions, it can pass) the life of the ATF is reduced causing problems to start.

ATF oxidizes, changes color (brown) and produces an odor similar to burnt bread. While heat destroys the lubricating qualities and friction characteristics of the fluid, at the same time, enamel begins to form on internal parts which interferes with the operation of the transmission.

If the fluid temperature exceeds 120°C (250°F), the rubber seals harden causing leakage and loss of pressure. Also at high temperatures, the transmission begins to slip, which further aggravates the heating. Eventually, the clutch burns out and the gearbox stops working. In the end, the only way to repair the damage is an overhaul, which comes at a considerable cost.

As a general rule, at every 6°C (20°F), the average increase in operating temperatures above 80°C will shorten the life of the transmission fluid to half. For example, at 91°C (195°F), the life of the fluid is reduced to 80,000 km. At 105°C (220°F) - which is the most common temperature in many transmissions - the ATF only works for 40,000 km and so the protection mileage is reduced.

Do not risk suffering a mishap related to the transmission fluid in automatics, make the change in the corresponding period and avoid paying more for not doing it.

Here's what BHPian vigsom had to say about the matter:

Very valuable information. Maybe this explains the guidelines set by Suzuki for automatic transmission fluid change intervals under different driving conditions (replacement at 1,65,000 km under normal driving and 30,000 km under severe driving). The ATF here is JWS3309 spec (Mobil ATF3309, Suzuki ATF3317) and one can extend this to Toyota T-IV too.

I've marked a red dot against ATF replacement recommendations in the following charts for quick reference.

Maintenance schedule - Normal driving conditions

Maintenance schedule - Severe driving conditions

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